Metal-Containing Poly(ionic liquid) Exhibiting Photogeneration of Coordination Network: Reversible Control of Viscoelasticity and Ionic Conductivity

The control of physical properties of soft matters via external stimuli is useful for various applications. Poly(ionic liquid)s are versatile liquid polymers exhibiting ionic conductivity, though the development of photoresponsive poly(ionic liquid)s is still limited. Here, we report the synthesis and properties of a photoreactive poly(ionic liquid) 1a consisting of a Ru sandwich complex [Ru(C5H5){C6H3(OC6H12CN)3}]+ and a polymeric anion -[-CH2–CH(SO2N–SO2CF3)-]n-. Upon UV photoirradiation, the liquid transformed into a rubbery elastomer 2a through the photochemical reaction of the cations. The resultant elastomer is a unique hybrid coordination polymer comprising a cationic coordination network and anionic covalent chains. The elastomer returned to 1a upon heating. The application of light and heat resulted in a coordination structure transformation that caused reversible changes in viscoelasticity and ionic conductivity. The liquid 1a was synthesized by polymerization of ionic liquid 1b containing an H2C═CHSO2N–SO2CF3 anion. UV photoirradiation transformed 1b into a viscous elastomer 2b consisting of a coordination network, though the elastomer changed into 1a upon heating via polymerization of the anion.